Time division (hereinafter referred to as TD) multiplexing (TDM: Time Division Multiplexing) is a technology in which a plurality of signal symbols occupying relatively narrow time durations share one relatively wide time duration in digital communication. Frequency division multiplexing FDM (Frequency Division Multiplexing) is a technology in which a plurality of signals occupying relatively narrow bandwidths share one relatively wide bandwidth. Used signal bandwidths are respectively B1, B2, B3, B4, . . . , or certainly, the signals may occupy a same bandwidth. AB is a minimum guard bandwidth, and an actual guard bandwidth may be wider. AB should be greater than a sum of a transition bandwidth of a used demultiplexing filter, a maximum frequency drift of a system, and a maximum frequency diffusion of a channel. This is a most common frequency division multiplexing technology. This technology is used in most existing systems such as broadcast systems, communications systems, and radar systems. A most significant feature of this technology is that used signal spectrums are isolated from each other, without mutual interference.
FIG. 1A is a schematic diagram of a conventional time division multiplexing technology. In FIG. 1A, time durations (referred to as timeslot widths in engineering) of multiplexed signal symbols are respectively T1, T2, T3, T4, . . . , and in engineering, the signal symbols usually occupy a same timeslot bandwidth. ΔT is a minimum guard timeslot, and an actual guard timeslot width should be larger. ΔT should be greater than a sum of a transition time width of a used demultiplexing gate circuit and a maximum time jitter of a system. This is a most common time division multiplexing technology. This technology is used in most existing systems such as multichannel digital broadcast systems and multichannel digital communications systems.
FIG. 1B is corresponding to a schematic diagram of a frequency division multiplexing technology. Used signal bandwidths are respectively B1, B2, B3, B4, . . . , or certainly, the signals may occupy a same bandwidth. ΔB is a minimum guard bandwidth, and an actual guard bandwidth may be wider. ΔB should be greater than a sum of a transition bandwidth of a used demultiplexing filter, a maximum frequency drift of a system, and a maximum frequency diffusion of a channel. This is a most common frequency division multiplexing technology. This technology is used in most existing systems such as broadcast systems, communications systems, and radar systems. A most significant feature of this technology is that used signal spectrums are isolated from each other, without mutual interference.
A most significant feature of this technology when it is applied to digital communications is: Multiplexed signal symbols are fully isolated from each other in terms of time, without mutual interference. The multiplexed signal symbols are not limited, and symbol durations (timeslot widths) of signals may have different widths. In addition, this technology is applicable to different communications mechanisms, provided that timeslots of the multiplexed signal symbols do not overlap or cross with each other. Therefore, this technology is most widely used. However, such multiplexing has no effect in improving spectrum efficiency of a system.
Therefore, a conventional idea is that adjacent channels do not overlap in a time domain, to avoid interference between the adjacent channels. However, this technology limits improvement of spectrum efficiency. An idea of a time division multiplexing technology in the prior art is that channels do not need to be isolated from each other and may strongly overlap with each other. As shown in FIG. 2A, in the prior art, overlapping between channels is considered as a new encoding constraint relationship, and corresponding modulation and demodulation technologies are proposed based on the constraint relationship. Therefore, a technology is referred to as an overlapped time division multiplexing (OvTDM: Overlapped Time Division Multiplexing). In this technology, spectrum efficiency increases proportionally with a quantity K of times of overlapping. In a frequency domain, a technology is correspondingly overlapped frequency division multiplexing (Overlapped Frequency Division Multiplexing). Correspondingly, this technology is shown in FIG. 2B.
Theoretically, when data transmission is performed by the overlapped time division multiplexing technology or overlapped frequency division multiplexing, the quantity K of times of overlapping may increase unlimitedly. Therefore, the spectrum efficiency may also increase unlimitedly. However, at a laboratory investigation stage, it is found that as the quantity K of times of overlapping increases, although the spectrum efficiency increases, transmit power also increases correspondingly. An increase in the transmit power also limits, in turn, an increase in the quantity K of times of overlapping to some extent, thereby limiting an increase in the spectrum efficiency.